UTM throughput would be the speed while "Unified Threat Management" options are enabled. Things like gateway anti-virus, web filtering, anti-spam, VPN tunnels, etc.
The "UTM throughput of 24 Mbps" you're quoting seems to be specific in the Netgear specs, in that it's referring to UTM options that affect HTTP traffic specifically (content filtering, etc.). AV speeds are covered in a separate number (and so on).
Regardless of how they've split it out in the documentation, in general the more UTM options enabled, the more processing of packets the box has to do on the fly, and in turn, the slower it will do it (due to the appliance's processing power, etc.).
If you are not intending on using UTM options like mentioned above, then the Firewall throughput would be the speed to expect (theoretically, on a good day, when the wind is blowing just right ;) ).